The dimerization domain of HNF-1 alpha: Structure and plasticity of an intertwined four-helix bundle with application to diabetes mellitus

Maturity-onset diabetes mellitus of the young (MODY) is a human genetic syn drome most commonly due to mutations in hepatocyte nuclear factor-1 alpha ( HNF-1 alpha). Here, we describe the crystal structure of the HNF-1 alpha di merization domain at 1.7 Angstrom, resolution and assess its structural pla sticity. The crystal's low solvent content (23 %, v/v) leads to tight packi ng of peptides in the lattice. Two independent dimers, similar in structure , are formed in the unit cell by a 2-fold crystallographic symmetry axis. T he dimers define a novel intertwined four-helix bundle (4HB). Each protomer contains two alpha -helices separated by a sharp non-canonical turn. Dimer -related alpha -helices form anti-parallel coiled-coils, including an N-ter minal "mini-zipper" complementary in structure, symmetry and surface charac teristics to transcriptional coactivator dimerization cofactor of HNF-1 (DC oH). A confluence of ten leucine side-chains (five per protomer) forms a hy drophobic core. Isotope-assisted NMR studies demonstrate that a similar int ertwined dimer exists in solution. Comparison of structures obtained in mul tiple independent crystal forms indicates that the mini-zipper is a stable structural element, whereas the C-terminal cc-helix can adopt a broad range of orientations. Segmental alignment of the mini-zipper (mean pairwise roo t-mean-square difference (rmsd) in C-alpha coordinates of 0.29 Angstrom) is associated with a 2.1 Angstrom mean C-alpha rmsd displacement of the C-ter minal coiled-coil. The greatest C-terminal structural variation (4.1 Angstr om C-alpha rmsd displacement) is observed in the DCoH-bound peptide. Diabet es-associated mutations perturb distinct structural features of the HNF-1 a lpha domain. One mutation (L12H) destabilizes the domain but preserves stru ctural specificity. Adjoining H12 side-chains in a native-like dimer are pr edicted to alter the functional surface of the mini-zipper involved in DCoH recognition. The other mutation (C20R), by contrast, leads to a dimeric mo lten globule, as indicated by its H-1-NMR features and fluorescent binding of 1-anilino-8-naphthalene sulfonate. We propose that a glycine-specific tu rn configuration enables specific interactions between the mini-zipper and the C-terminal coiled-toil. (C) 2001 Academic Press.